This invention relates to an apparatus for automatically orienting a wafer of semiconductive material to a desired position relative to the distinctive peripheral feature of the wafer.
In the manufacture of solid state devices such as transistors and integrated circuits on a single semiconductive wafer using a multistep photomechanical reproduction process, each of the successive photoresist mark alignment and fabricating steps requires that the wafer be accurately placed in a predetermined orientation position at a predetermined location. In more detail, a wafer is given to apply on its surface a photosensitive layer made of such as Photo Resist, and is then transferred to a printing station where the surface is selectively exposed through a photomask to define a plurality of impurity diffusion areas. This photomask printing operation is performed in alignment of the photomask with the wafer, after the randomly oriented wafer has been oriented to a desired position at a predetermined location on a support plate. To impart such orientation to the wafer, a distinctive physical feature has to be formed in the periphery thereof to serve as the orientation reference. For example, a linear or recessed peripheral portion is distinctively formed with respect to the other round peripheral portion, the linear portion being commonly called "orientation-flat" or "orientation-cut", and the recessed portion "orientation-notch". As a general term for these terms will be hereinafter used "orientation reference portion". It has been the practice of the prior art in the orientation of each wafer that the operator manually brings the orientation reference portion into engagement with a certain orientation abutment member in the orientor station. The inclusion of manual orienting steps in the manufacture of the solid state devices on a semiconductive wafer not only considerably slows the overall production capability, but also reduces the accuracy of orientation control depending upon the personal experience of the operator.
In order to eliminate the above-mentioned drawbacks, a solution has been proposed as, for example, in an automatic wafer orienting apparatus disclosed in Japanese open Pat. No. Sho 47-24775. This apparatus is adapted to orient a wafer having an orientation-cut, employing a plurality of guide rollers arranged to define an area of rotation of the wafer about its axis as it is rotated by a rotary support plate, and an orientation sensor connected to the drive means for the rotary support plate upon interaction with the orientation-cut to automatically terminate the driving of the rotary support plate, whereby the orientation of the wafer to a desired position is effected. A major problem of this apparatus is that, due to the fragility of the semiconductive wafers which is particularly serious for silicon wafers each having a thickness of about 0.04 cm and a diameter of more than 2.5 cm, the guide rollers as arranged along the circumference of the wafer destroy many wafers by cracking, chipping-off, or imparting stress to the wafer body, as the wafer is rotated while permitting the orientation-cut to strike some or all of the guide rollers. For this reason, the apparatus of the above-identified Japanese open patent is not suited for use in the manufacture of solid state devices using a multistep photomechanical reproduction process.
To minimize the probability of destroying semiconductive wafers, it is desirable to decrease the number of guide rollers which engage with the periphery of the wafer. In this respect, there is known another type automatic wafer orienting apparatus employing only one guide roller and a pair of reversely rotatable rotary heads, the arrangement of these parts being such that when the orientation-cut of the wafer is brought into contact with both of the rotary heads, the wafer is held from rotation in a desired orientation position. With this apparatus, however, the reverse rotatative movements of these two rotary heads are simultaneously applied to the periphery of the wafer causing the wafer to vibrate at any time so that the wafer is unstable in position during the orientation operation. In addition to such vibratory motion of the wafer, a difference between the friction coefficients of the two rotary heads for the periphery of the wafer and other various factors make it difficult to control the orientation position and location with high accuracy and reliability.